加減消元法

我都系E本書BO~
1)﹛5x+3y=10 ---(1)
x+2y=9 ----(2)
from(2)
x=9-2y ----(3)
sub (3)to(1)
5(9-2y)+3y=10
45-10y+3y=10
-7y=10-45
-7y=-35
y=5

sub y=5 into(2)
x+2y=9
x+2(5)=9
x+10=9
x=10
thus, the solution is x=10, y=5

2)﹛10x-3y=69 -----(1)
2x-9y=39----(2)
from(2)
2x=39+9y
x=39+9y
--------- ----(3)
2
sub(3)into (1)
10(39+9y)-3y=69
--------
2
5(39+9y)-3y=69
195+45y-3y=69
195+42y=69
42y=69-195
42y=-126
y=-3
suby=-3 into(1)
10x-3y=69
10x-3(-3)=69
10x-(-9)=69
10x+9=69
10x=60
x=6
thus, the solution is x=6, y=-3
3)
15x+7y=-5 ...1
11x+10y=-28 ...2
10*1,
150x+70y=-50 ...3
7*2
77x+70y=-196 ...4
3-4
73x=146
x=2
x=2into 2
11(2)+10y=-28
22+10y=-28
10y=-50
y=-5
thus,the solution isx=2, y=-5

1)
5x+3y=10 ...1
x+2y=9 ...2
5*2,
5x+10y=45 ...3
3-1
7x=35
x=5
x=5 put into 1
5(5)+3y=10
25+3y=10
3y=-15
y=-5
so
(x,y)=(5,-5)

2)
10x-3y=69 ...1
2x-9y=39 ...2
5*2,
10x-45y=195 ...3
3-1
-42y=126
y=-3
y=-3 put into 2
2x-9(-3)=39
2x+27=39
2x=12
x=6
so
(x,y)=(6,-3)

3)
15x+7y=-5 ...1
11x+10y=-28 ...2
10*1,
150x+70y=-50 ...3
7*2
77x+70y=-196 ...4
3-4
73x=146
x=2
x=2 put into 2
11(2)+10y=-28
22+10y=-28
10y=-50
y=-5
so
(x,y)=(2,-5)

sol
1. 5x+3y=10...(1)
x+2y=9...(2)
(1)*2-(2)*3
=(10x+6y)-(3x+6y)=7x
=10*2-9*3=20-27=-7
x=-1
sub x into (2),2y=10
y=5
2. 10x-3y=69...(1)
2x-9y=39...(2)
(1)*3-(2)
=(30x-9y)-(2x-9y)=28x
=69*3-39=246
28x=246,thus x=6
sub x into (2),12-9y=39
iq. -9y=27
y=-3
3.
15x+7y=-5...(1)
11x+10y=-28...(2)
(1)*10-(2)*7
=(150x+70y)-(77x+70y)=73x
=(-5)*10-(-28)*7=146
x=2
sub x into (2) then
22+10y=-28
10y=-40
y=-4

2011-11-04 17:58:46 補充：
Corr
2.(1)*3-(2)
=(30x-9y)-(2x-9y)=28x
=69*3-39= 168
x=168 / 28 = 6

2011-11-04 17:59:20 補充：
3.
sub x into (2) then
22+10y=-28
10y=-50
y=-5

2011-11-07 21:10:16 補充：
以後請先告訴我,否則後果自付